This invention relates to an emulsion polymerization process and more particularly to a starve fed emulsion polymerization process. Such process is particularly useful in the production of in situ and conventional toner resins, wherein it is desired to control both the molecular weight and the molecular weight distribution of the resultant toner resin.
The resins produced by the starve fed emulsion polymerization process may be utilized for making dry electrostatographic toners according to conventional methods. Such toners may then be used in conventional electrostatographic imaging processes. The resins produced according to the described methods may also be used in liquid developers and inks, for example, for ink jet applications.
Conventional toner resins and methods for producing such resins are known in the art.
Starve fed emulsion polymerization processes are also known in the art, although not for producing toners.
For example, Li and Brooks, "Semi-batch Processes for Emulsion Polymerization," Polymer International, Vol. 29, No. 1, pp. 41-46 (1992), presents a general discussion and analysis of starve fed emulsion polymerization. Li and Brooks discuss both neat monomer feed, where straight monomer is introduced into the reactor with an emulsifier, and emulsion feed, where the monomer or monomers are pre-emulsified before introduction into the reactor. In both cases, the remaining straight or emulsified monomer is fed into the reactor in a starved state.
As a further example, U.S. Pat. No. 4,628,071 discloses a semicontinuous emulsion copolymerization process for producing polymer of vinyl and acrylate monomers. The process involves the semicontinuous starved feed of a mixture of the monomers to a reactor containing a precharge of an acrylic acid monomer. The polymers are disclosed as useful in inks, floor finishes, coatings and adhesives.
Variants of starved feed emulsion polymerization processes have been applied most widely in the preparation of latexes. For example, U.S. Pat. Nos. 4,946,891 and 3,498,938 disclose processes for preparing latexes useful, for example, in paints.
Polymeric resins suitable for applications in electrostatographic toners may be made by a variety of polymerization techniques. In particular, vinyl polymer resins may be made by free radical polymerization methods, including built polymerization, solution polymerization, suspension polymerization, emulsion polymerization, anionic polymerization and cationic polymerization. Each of these methods has its individual advantages and limitations.
Bulk polymerization comprises initiation of polymerization in pure or nearly pure (about 100%) monomer. Due to the large amounts of heat generated by the built polymerization process, and due to the poor heat transfer as the mixture viscosity builds up during the polymerization, this is the most dangerous type of reaction. Sometimes bulk polymerization leads to explosions.
Solution polymerization involves diluting the monomer with a suitable solvent in order to maintain a lower viscosity during the polymerization process. Solution polyermized polymers are characterized by relatively low molecular weight (Mw=10,000 to 100,000) and molecular weight distribution (MWD=2 to 4). The molecular weight can be lowered by adding a chain transfer agent, but this usually causes the MWD to increase. This occurs because reactive chain transfer agents are consumed quickly in the early part of the polymerization, giving rise to low molecular weight polymer at the beginning. In the later stages of the polymerization reaction, there is no chain transfer agent left, and the instantaneous molecular weight being formed is greater. Thus, the MWD is larger. It is difficult to achieve low molecular weight and low molecular weight distribution, or high molecular weight and high molecular weight distribution. Also, the presence of solvent can lead to incomplete polymerization, and high residual monomer (greater than 1%). Residual monomer may pose health hazards to the end user, or may produce objectionable smells.
A further disadvantage of solution polymerization, and indeed, of all batch polymerization reactions, is the problems caused by unequal reactivity in copolymerizations. Because mixtures of monomers may differ in reactivity, the more reactive monomer may polymerize more quickly at the beginning of a reaction, becoming consumed more rapidly, and thus leaving the polymer at the end of the reaction comprised more heavily of the more slowly reacting monomer. Such well known compositional drift generally impairs the properties of the final resin, and may even cause two or more incompatible phases to be formed. This limits the pairs of monomers which may be copolymerized in solution and in other bulk polymerization processes.
Suspension polymerization comprises forming monomer droplets suspended in water, then polymerizing the monomer in the presence of an oil soluble initiator. The molecular weight properties are comparable to the bulk polymerization described above, except that the heat transfer out of the mini bulk droplet reactors is better. The molecular weight and residual monomer situations are comparable to the bulk and solution polymerizations.
Emulsion polymerization comprises forming an emulsion of a surfactant and monomer in water, then polymerizing the monomer in the presence of a water soluble initiator. Emulsion polymerization is a well known industrial process. In most prior art embodiments, emulsion polymerization is used to make very high molecular weight polymers (100,000 to several million) with low MWD (2 to 4) via batch emulsion polymerization. The molecular weight is very high because the particles are so small that they encounter initiator very rarely. Furthermore, the high interior viscosity promotes a gel effect, whereby the growing chains have difficulty terminating because the radicals cannot move toward each other before growing to great length. An advantage of emulsion polymerization is the low residual monomer which may be achieved under favorable circumstances (50 to 1,000 ppm) due to high conversion. Chain transfer agents can be used to decrease the molecular weight, but added chain transfer agent also greatly increases the MWD. Thus, it is not possible for batch emulsion polymerization to generate resins of low Mn and low MWD, or of high Mn and high MWD.
Another undesirable characteristic of conventional batch emulsion polymerization reactions is that such batch reactions have inherently poor reproducibility from reaction to reaction. It is believed that such poor reproducibility is due to the strong exotherms which are characteristic of the batch reactions.
Among ionic polymerization methods are anionic and cationic polymerization. In these cases, all the chains are initiated simultaneously and grow together, resulting in very narrow molecular weight distributions (MWD=1 to 2), and low to moderate molecular weights (Mn=500 to 1,000,000). These reactions must be conducted at very low temperature (-80.degree. C. to 0.degree. C.) and in highly purified solvents. Therefore, ionic polymerization processes are not always economical on the large scale required for electrostatographic toners. It is generally not possible to broaden the molecular weight distribution using these anionic and cationic polymerization processes.
As described above, a problem with conventional polymerization processes is that they do not allow the simultaneous control of the number average molecular weight (Mn) and the molecular weight distribution (MWD) of a toner resin. Simultaneous control of those parameters is desired to provide optimal resin characteristics, such as temperature fixability, glass transition temperature, gloss qualities, and solid area density. However, current processes do not allow simultaneous control of Mn and MWD over the entire Mn-MWD domain. Efforts to improve upon the characteristics of toner resins have therefore been hampered by the inability to create toner resins within the full Mn-MWD domain.